As used herein, the term "film" refers to a thin, flexible, single- or multi-layer polymeric sheet. The term is used interchangeably with the terms "backing" and "carrier web". Graphic marking films or labels formed from vinyl films coated with acrylic pressure sensitive adhesives (PSAs) are well-known in the art. However, to date, the ability to provide such label and graphic marking film films with high bonding strength, (i.e. non-removal), to low energy surfaces such as high density polyethylene and polypropylene plastics has not been possible without sacrificing important properties of the adhesive such as shear strength and cohesive strength.
As used herein, the term "low energy surfaces" is intended to mean those surfaces which exhibit low polarity and low critical surface tension (less than about 40 dynes/cm.sup.2) characteristics. One example of a low energy surface is the surface of a polyolefin plastic. Among the PSAs, it is known that acrylic-based PSAs exhibit poorer bond characteristics to low surface energy polyolefin plastics than do rubber-based PSAs. This effect results from the greater difference in polarity between the acrylic PSA and the polyolefin surface as compared to that between the rubber PSA and the polyolefin surface. Unfortunately, however, it would be very desirable to use acrylic PSAs in many applications, since acrylic PSAs exhibit excellent outdoor durability, whereas rubber PSAs show poor ultraviolet and oxidative stability due to chemical unsaturation of the hydrocarbon elastomer.
Common physical methods to obtain high bond strength of pressure sensitive adhesives to polyolefin plastics include flame treating, which oxidizes the surface of the plastic, chemical etching with strong acids to increase polarity of the bonding surface, or the use of a primer or topcoat containing a chlorinated polyolefin. For example, Japanese Patent No. HEI 1(1989)-242676 discusses the use of chlorinated polyolefin resins in connection with pressure sensitive adhesives. One disadvantage of such surface treatment methods is that they are inefficient in that they add an additional process step when applying graphic marking films or labels to low energy surfaces. This is less than ideal since industrial consumers of graphic marking films and labels desire that these products can be easily applied in a single step without the need for additional, time consuming surface preparation methods.
For a permanent graphic marking film or label application, it would be desirable to have bond characteristics similar to those associated with rubber-based PSAs combined with the outdoor durability associated with acrylic PSAs. One method to increase bond strength of acrylic PSAs to low surface energy polyolefin plastics is to incorporate a compatible tackifier such as a rosin ester, a terpene phenolic resin or a hydrocarbon resin into the adhesive. Although the use of a tackifier dramatically improves bond strength as measured by peel force at low speeds, inclusion of these tackifiers raises the glass transition temperature (Tg) of the PSA which results in reduced low temperature performance and also causes a "shocky" or "zippy" peel characteristic at faster peel rates. This "shocky" or "zippy" peel is an undesirable characteristic which can result in easy removal of films and labels, as well as making them less tamper-resistant.
Additionally, loss in adhesive shear strength and cohesive strength is also observed if large amounts of tackifier or plasticizer are incorporated into the PSA.
In many applications, it is desirable that graphic marking films or labels be difficult to remove once they are applied to a surface. This may be accomplished by providing the graphic marking film or label film with a means by which attempts to remove it will result in tearing or other damage to the graphic marking film or label. A method to increase the destructibility of PSA coated vinyl labels or graphic marking films is to make the vinyl film backing less elastic or "brittle". This is achieved by adding a hard acrylic resin and decreasing the plasticizer level of the vinyl film.
U.S. Pat. Nos. 5,141,790 (Calhoun et al.) and 5,296,277 (Wilson et al.) the teachings of both of which are incorporated herein by reference, describe an adhesive film referred to commercially as Controltac Plus.RTM. film (available from Minnesota Mining and Manufacturing Company, hereafter "3M"). The adhesive surface of the Controltac Plus.TM. film is characterized in that it includes clustered domains of a non-adhesive material, referred to as "pegs" which extend a short distance from the adhesive surface. The patents also describe adhesive films in which the adhesive surface is microtextured or provided with a microtopological structure.